268 Heat Stress Impacts on Cattle Production

Abstract

Abstract Heat stress (HS) compromises farm animal productivity and it is thus both an economic and food security issue. Reduced output (e.g., milk yield, muscle growth, egg production) during HS was traditionally thought to result from decreased nutrient intake (a classic biological response shared by all animals during environmental-induced hyperthermia). Utilizing pair-feeding experimental designs clearly indicate that reduced feed intake only accounts for a portion of the decreased productivity. The etiological epicenter of many (if not most) of the physiological consequences of HS is gastrointestinal tract hyperpermeability. For reasons not clearly understood, HS compromises the intestinal barrier allowing luminal antigens to infiltrate into local and systemic circulation. The invading antigens are recognized and stimulate the immune system. An activated immune system rearranges the hierarchy of nutrient partitioning, and the entire body participates in ensuring that leukocytes receive the fuel and building blocks necessary to mount an effective response. Central to this is increased insulin secretion coupled with simultaneous decreased adipose tissue and skeletal muscle insulin resistance. Activated leukocytes switch their energetic metabolism away from oxidative phosphorylation to aerobic glycolysis. Utilizing a LPS-Euglycemic clamp we have estimated that an intensely activated immune system can use more than 2 kg of glucose/d in lactating cows. We extended this in growing steers and pigs and the amount of glucose used is approximately 1 g/Kg BW0.75/h regardless of species and physiological state. If the heat load is severe enough, the consequences can become lethal. This is primarily explained by inflammation induced hypercoagulation, endothelial barrier dysfunction and disseminated intravascular coagulation. If the animal survives, these changes result in the reprioritization of fuel selection during HS which appears to be primarily responsible for reduced animal productivity during the warm summer months.